Semiconductor transistors, in particular field-effect controlled switching devices such as a MISFET (Metal Insulator Semiconductor Field Effect Transistor), in the following also referred to as MOSFET (Metal Oxide Semiconductor Field Effect Transistor) and a HEMT (high-electron-mobility Field Effect Transistor) also known as heterostructure FET (HFET) and modulation-doped FET (MODFET) are used in a variety of applications. An HEMT is a transistor with a junction between two materials having different band gaps, such as GaN and AlGaN.
In a GaN/AlGaN based HEMT, a two-dimensional electron gas (2DEG) arises at the interface between the AlGaN barrier layer and the GaN buffer layer. In an HEMT, the 2DEG forms the channel of the device instead of a doped region, which forms the channel in a conventional MOSFET device. Similar principles may be utilized to select buffer and barrier layers that form a two-dimensional hole gas (2DHG) as the channel of the device. A 2DEG or a 2DHG is generally referred to as a two-dimensional carrier gas. Without further measures, the heterojunction configuration leads to a self-conducting, i.e., normally-on, transistor. Typically, measures must be taken to prevent the channel region of an HEMT from being in a conductive state in the absence of a positive gate voltage.
HEMTs are viewed as an attractive candidate for power transistor applications. A power transistor is a device that is capable of switching substantial voltages and/or currents associated with high power applications. For example, a power transistor may be required to block a voltage of at least 200 V, 400 V, 600 V or more. In addition, a power transistor may be required to conduct currents in the range of ones, tens or hundreds of amperes during normal operation. Due to the high electron mobility of the two-dimensional carrier gas in the heterojunction configuration, HEMTs offer high conduction and low losses in comparison to many conventional semiconductor transistor designs and therefore are well suited for these large operating currents.
Known HEMT designs have a number of limitations that detrimentally impact their suitability for power transistor applications. One limitation of GaN technology relates to the breakdown strength of a GaN/AlGaN based HEMT. Conventionally, the breakdown strength of a GaN/AlGaN based HEMT can be improved by increasing the thickness of the GaN buffer layer. However, this technique introduces cost and complexity to the manufacturing process. Other issues with HEMT designs include poor dynamic on-resistance, current collapse and reliability concerns, e.g., due to the high electric fields between source and drain fingers at the surface of the devices due to the inherent lateral structure of the HEMT.